I'm gonna go ahead and say the
Atlas, at least the AS7-D. While it's certainly nothing to sneeze at... on paper, the problem is that it's almost strictly a short-range combatant once it runs out of LRM ammo. If it's put into a lengthy long-range sniping duel with, say, an
Awesome, chances are that sooner or later it will become for all intents and purposes a mobile, heavily-armored, slow-moving 100-ton paperweight.
I understand what Kerensky was thinking when he wanted a big, ugly bruiser that would scare the snot out of anything on the field, but what was he smoking when he dreamed up a big, ugly bruiser that had only one (and an iffy one at that!) long-range weapons system?
Albatross
When anything is named "albatross", the jokes write themselves.